Post-inflammatory hyperpigmentation (PIH) is an acquired hypermelanosis that occurs after cutaneous inflammation or injury that frequently affects darker skinned populations. Previously, a model of 35% TCA-induced PIH was validated against acne induced PIH, which has value in product testing for the treatment of PIH. In this second phase of the study, the investigators would like to determine if a lower concentration of TCA-induced PIH is comparable to acne-induced PIH.

Post-inflammatory hyperpigmentation (PIH) is an acquired hypermelanosis that occurs after cutaneous inflammation or injury. This process can occur in all skin types but more frequently affects darker skinned patients, such as African-Americans, Hispanics, Asians, Native Americans, Pacific Islanders and those of Middle Eastern descent. PIH can occur after infection, allergic reactions, contact dermatitis, some medications, burns, following procedures, or inflammatory disease such as acne. In skin of color, PIH frequently occurs in resolving acne lesions and can persist for months after the acne lesion itself has disappeared. In many cases, the resulting PIH can be more distressing than the original insult.

During the first phase of this study, the investigators investigated the clinical, spectroscopic and histologic characteristics of acne-induced PIH versus irritant induced PIH using Trichloroacetic acid (TCA), 35% solution. From this initial study, the investigators concluded that the similarity of Investigator's Global Assessment scores, and spectroscopic measurements using Diffuse Reflectance Spectroscopy and Colorimetry in both acne and TCA-induced PIH at Day 28 suggest that TCA-induced PIH could be a reproducible model for acne induced PIH.

MicroRNAs (miRNAs) are a class of small non-coding RNA molecules that regulate the expression of multiple genes at the post-transcriptional level through degradation and translation of target mRNAs. In the initial study, the investigators hypothesized that miRNAs derived from melanocytes and immune cells during PIH development could be detected in tissue and serve as novel biomarkers for PIH and making appropriate therapeutic decisions. To test this hypothesis, the investigators first examined miRNA gene expression profiles during PIH development using different models, and then evaluated miRNAs profiles in acne- induced PIH, TCA- induced PIH and normal skin. The investigators have defined some miRNAs that potentially are involved in PIH development and may be also serve as the biomarkers for PIH. The investigators found that there were 19 miRNA changes in acne-induced PIH versus normal skin, while 43 miRNA changes in TCA-inducedPIH versus normal skin. Interestingly, about 80% changed genes in acne were included in TCA-mediated miRNA changes, suggesting TCA can partially mimic acne-PIH.

Overall, this initial model for PIH, using TCA, serves as a foundation to better understand and improve our ability to manage PIH. In this next phase of the study, the investigators will refine this in vivo model for PIH by determining the optimal concentration of TCA to induce PIH.

Inclusion Criteria:

• Patients with types I-VI skin
• Minimum age of 18 years
• Able to understand requirements of the study and risks involved
• Able to sign a consent form.
• Existing truncal acne pustules (at least two on the trunk) with or without history of post-inflammatory hyperpigmentation

Exclusion Criteria:

• Patients with a recent history of vitiligo, melasma, and other disorders of pigmentation with the exception of PIH judged to be clinically significant by the investigator
• Patients with a history of keloids
• Patients with a history of cystic acne or acne conglobata
• Patients on systemic antibiotics or keratolytics (isotretinoin, etc), or topical antibiotics or keratolytic use (retinoids, benzoyl peroxide) over target areas who are unwilling to stop these medications for the duration of the study.